The American flag flies at half-staff outside the Pentagon on Tuesday morning. Photo: Suzanne Kennedy

A number of events will be held to commemorate the dark day in American history.

The District of Columbia is marking the 11th anniversary of the Sept. 11 terrorist attacks with an "afternoon of service" in Freedom Plaza.

District officials and city government workers will gather in the plaza along Pennsylvania Avenue near the White House for service projects intended to support veterans and first responders.

The organization Greater D.C. Cares has recruited thousands of volunteers for it s United We Serve day of service Tuesday. The volunteers will mark 9/11 by helping out at more than 50 non-profits throughout the Washington region.

D.C. Councilmember Mary Cheh will visit several fire stations that responded to the Pentagon after the Sept. 11 terror attacks. Cheh is delivering refreshments to several fire houses in her ward that helped with Pentagon evacuation efforts.

President Barack Obama, first lady Michelle Obama and the White House staff will mark a moment of silence on the south lawn of the White House.

The president and the first lady will also attend a Sept. 11 memorial event at the Pentagon and visit service members at the Walter Reed National Military Medical Center in Bethesda.

Defense Secretary Leon Panetta will host a remembrance ceremony at the Pentagon memorial. Congress will also hold a Sept. 11 remembrance ceremony.

A program will be held in Shanksville, Pa. at the Memorial Plaza, with keynote remarks from Vice President Joe Biden. Secretary of the Interior Ken Salazar will also attend.

In New York City, a program will be held at 8:39 a.m. at the National September 11 Memorial Plaza at the World Trade Center site, with bagpipers and drummers.

Moments of silence will mark pivotal moments of that tragic day:

8:46 a.m. - Moment of silence marking when American Airlines Flight 11 hit the north tower, and houses of worship toll their bells. Then family members will read the names of those killed.

9:03 a.m. - Moment of silence marking when United Airlines Flight 175 hit the south tower.

Monday, the organization Muslims for Life hosted its annual blood drive on Capitol Hill to honor Sept. 11 families.

Flags can been seen flying throughout the Washington region for Tuesday's anniversary, including in Arlington and along the 14th Street Bridge. In a statement released Tuesday morning, Republican presidential nominee Mitt Romney said America will never stop caring for the people who lost loved ones in the attacks.

"Today we again extend our most profound gratitude to our brave troops who have gone into battle, some never to return, so that we may live in peace," Romney said.

William Francis and his wife, Shan, came to the National Cathedral Monday to pray for those who serve, including their son, who is in the Navy. Shan says her son was overseas on Sept. 11, 2001.

"He said, 'Don't worry about me. I'm worried about you,' and he said, 'Carry on and do everything that you are doing in your community, because that is why we are here to serve,'" Shan recalls.

William adds, "You know where you were at the time, just like I know where I was when Kennedy was assassinated."

Jana Hogg's school, Wesley Theological, will offer special prayers Tuesday. Hogg was an Oklahoma City sophomore eleven years ago in Sept. She knew the pain families affected by 9/11 would suffer, remembering the 1995 Murrah building bombing.

"It triggered a lot of memories from that, because I was in fourth grade when that had happened," Hogg says.

Isaac McAslin remembers writing condolence letters to 9/11 victims in first grade. Now a freshman at George Washington University, he participated in the university's annual day of service to commemorate 9/11.